This application claims the benefit of Korean Patent Application No. 1999 20861, filed on Jun. 5, 1999, which is hereby incorporated by reference.
1. Field of the Invention
This invention relates to a reflective liquid crystal display device, more particularly to a reflective liquid crystal display device exhibiting high brightness, good color characteristics and an improved contrast ratio with high productivity.
2. Description of Prior Art
In general, liquid crystal displays are divided into a transmissive or reflective liquid crystal display depending on whether it uses an internal or an external light source.
While a transmissive liquid crystal display device uses an internal light source such as a backlight, a reflective liquid crystal display device uses ambient light and thus is affected by the environment. For example, the, brightness of light in an office differs largely from that in the street. Also, the brightness of ambient light in a place can vary during different times of a day.
Because of these limitations of a reflective liquid crystal display device, there has been an urgent need for developing a reflective liquid crystal display that utilizes the external light to the maximum and has an improved contrast ratio. Examples of liquid crystal modes for embodying such reflective liquid crystal displays include a twisted nematic LC mode, a polymer-dispersed LC mode and a guest-host LC mode. Of these, the twisted nematic LC mode is most desirable for practical reasons.
A reflective liquid crystal display using the optical properties of a twisted nematic liquid crystal mode is generally based on the following principle.
FIG. 1 is a cross-sectional view showing the structure of a general reflective liquid crystal display device, which is comprised of a twisted nematic (TN) liquid crystal (LC) material 14 sandwiched between a transparent substrate 18 and an opposing substrate 10 having a reflector 12. A common electrode 16 of a transparent metal, positioned between the liquid crystal layer 14 and the transparent substrate 18, applies an electric field on the LC layer 14. On the transparent substrate 18, a retardation film 20 is disposed to convert a linearly polarized light to a circularly polarized light. A linear polarizer 22, which converts natural light to a linearly polarized light, is positioned on the retardation film 20. FIG. 2:1 is a diagrammatic view illustrating transformations of the incident light when no voltage is applied, and FIG. 3 is also a diagrammatic view but illustrating the transformations of the incident light when a voltage is applied.
When no voltage is applied, as shown in FIG. 2, the incident light is converted to a linearly polarized light through the linear polarizer 22, and then converted to a circularly polarized light (assumed to be a left-handed circularly polarized light) through the retardation film 20.
The circularly polarized light then passes through the liquid crystal layer 14 and is converted to a linearly polarized light, then is reflected on the reflector 12.
The reflected light again passes through the liquid crystal layer 14 and is converted to a left-handed circularly polarized light, then after passing through the retardation film 20, the circulary polarized light is: converted to a linearly polarized light parallel to a vertical axis of the linear polarizer.
In contrast, when a voltage is applied as shown in FIG. 3, the incident light is converted to a linearly polarized light through the linear polarizer 22, then to a circularly polarized light (assumed to be a left-handed circularly polarized light) through the retardation film 20.
The light passes through the liquid crystal layer 14 with no change and is reflected on the reflector 12 to become a right-handed circularly polarized light. Then, it passes through the liquid crystal 14 with no change and through the retardation film 20 to become a linearly polarized light parallel to the vertical axis of the linear polarizer. The light is absorbed by the polarizer at this point.
In the above described reflective LCD device, the brightness, contrast ratio and color characteristics depend on the twist angle of the LC layer and on a retardation dxcex94n (where xcex94n is the anisotropy of the refractive index of the LC material, and d is the spacing between substrates.)
U.S. Pat. No. 4,019,807 discloses that for a reflective liquid crystal light valve, an optional optimal twist angle is 45xc2x0. For such a twist angle, the maximum liquid crystal mode efficiency (defined later) can be obtained when retardation dxcex94n is about 0.16 xcexcm or 0.38 xcexcm.
However, when dxcex94n=0.16 xcexcm, the cell gap is too thin for use in an actual process (making the ease of manufacture or manufacturability poor), whereas when dxcex94n=0.38 xcexcm, the yield is good, but the color characteristic, contrast ratio and viewing angle property are not good due to dispersion.
Also, the above property changes greatly with minor variation of dxcex94n (or dxcex94n variation tolerance). Thus, the processing margin of error becomes very small. That is, when the twist angle 45xc2x0, minor variation to dxcex94n result in great changes to the above-discussed properties. Thus, applying the 45xc2x0 twist angle in actual processing becomes difficult.
To mitigate these problems, U.S. Pat. No. Re. 35, 799 discloses that the twist angle and dxcex94n value can be set to 63xc2x0 and 0.20 xcexcm to obtain a higher reflectance ratio and a better color characteristic, as well as to obtain increased yield due to the increase in cell gap.
The reflective liquid crystal display in the above patent uses the modified twist angle and dxcex94n value for quality improvement. However, the cell gap to meet the dxcex94n of 0.2 xcexcm is still too thin. To achieve satisfactory production using current technique, the cell gap should be 3.0 xcexcm or greater. The dxcex94n value of TN LC material is 0.068. With such a material, the cell gap will be 2.94 xcexcm to achieve dxcex94n of 2.0 xcexcm. This cell gap value is less than the practical limit of 3.0 xcexcm, and thus the yield is likely to be lower than satisfactory.
Another shortcoming of these values is that the voltage applied for a white display mode will be high, leading to a low contrast ratio.
Therefore, there is still the need to modify the twist angle and dxcex94n to assure good yield and a high contrast ratio.
To restate, satisfactory yield in the conventional art have been achieved, but these devices exhibit unsatisfactory display characteristics, e.g., liquid crystal mode efficiencies, and dxcex94n variation tolerances. Conversely, satisfactory liquid mode efficiencies have been achieved in the conventional art, but the yield have been unsatisfactory.
The invention is, in part, a recognition that a liquid crystal display device can be produced that exhibits satisfactory display characteristics, e.g., liquid crystal mode efficiencies and dxcex94n variation tolerances, while at the same time exhibit satisfactory yield. This is unexpected given the conventional art.
In order to achieve the objects, the present invention provides, in one aspect, a reflective liquid crystal display device having: first and second substrates spaced apart and facing each other; a twisted nematic liquid crystal layer sandwiched between the two substrates, the liquid crystal layer having a twist angle and birefringence property xcex94n and having a thickness d; a retardation film: disposed on the first substrate and opposite to the liquid crystal layer, the film converting a first linearly polarized light to a first circularly polarized light and converting a second circularly polarized light to a second linearly polarized light; a linear polarizer disposed on the retardation film and opposite to the liquid crystal layer and converting the incident light to the first linearly polarized light; and a reflector disposed between the liquid crystal layer and the second substrate and reflecting light passed through the liquid crystal layer, wherein an optimized condition comprises a twist angle range between about 65xc2x0 and 75xc2x0 with a dxcex94n in a range between about 0.20 xcexcm and 0.28 xcexcm.
In another aspect, the present invention provides a reflective liquid crystal display device includes: first and second substrates spaced apart and facing each other; a twisted nematic liquid crystal layer sandwiched between the two substrates, the liquid crystal layer having a twist angle and birefringence property xcex94n and having a thickness d; a retardation film disposed on the first substrate and opposite to the liquid crystal layer, the film converting a first linearly polarized light to a first circularly polarized light and converting a second circularly polarized light to a second linearly polarized light; a linear polarizer disposed on the retardation film and opposite to the liquid crystal layer and converting the incident light to the first linearly polarized light; and a reflector disposed between the liquid crystal layer and the second substrate and reflecting light passed through the liquid crystal layer, wherein an optimized condition comprises a twist angle range between about 75xc2x0 and 85xc2x0 with a dxcex94n in a range between about 0.18 xcexcm and 0.26 xcexcm.
Objectives of the present invention will become more apparent from the detailed description given hereinafter. However, it should be understood that the detailed description and specific examples, while indicating preferred embodiments of the invention, are given by way of illustration only, since various changes and modifications within the spirit and scope of the invention will become apparent to those skilled in the art from this detailed description.